In a Direct-Sequence/Spread-Spectrum (DS/SS) system, a RAKE receiver is used to improve a bit error rate (BER) performance. The RAKE receiver can collect more signal energy through independent paths and achieve path diversity. The RAKE receiver obtains further diversity gain through fractional sampling. However, the power consumption of the RAKE receiver increases in proportion to a sampling rate and does not always maximize the signal-to-noise ratio (SNR). Therefore, sampling rate selection schemes have been proposed to reduce the average sampling rate without degrading the BER. These schemes select the tap positions and the sampling rate depending on channel conditions and the power consumption can be reduced. In this paper, sampling rate selection schemes for the DS/SS system are investigated through an experiment since there have been no numerical results through an experiment. Numerical results show that the power consumption can be reduced even through the experiment without the degradation of the BER.

This paper proposes an enhanced method for estimating the a priori Signal-to-Disturbance Ratio (SDR) to be employed in the Acoustic Echo and Noise Suppression (AENS) system for full-duplex hands-free communications. The proposed a priori SDR estimation technique is modified based upon the Two-Step Noise Reduction (TSNR) algorithm to suppress the background noise while preserving speech spectral components. In addition, a practical approach to determine accurately the Echo Spectrum Variance (ESV) is presented based upon the linear relationship assumption between the power spectrum of far-end speech and acoustic echo signals. The ESV estimation technique is then employed to alleviate the acoustic echo problem. The performance of the AENS system that employs these two proposed estimation techniques is evaluated through the Echo Attenuation (EA), Noise Attenuation (NA), and two speech distortion measures. Simulation results based upon real speech signals guarantee that our improved AENS system is able to mitigate efficiently the problem of acoustic echo and background noise, while preserving the speech quality and speech intelligibility.

Although a distributed index on a distributed hash table (DHT) enables efficient document query processing in Peer-to-Peer information retrieval (P2P IR), the index costs a lot to construct and it tends to be an unfair management because of the unbalanced term frequency distribution. We devised a new distributed index, named Huffman-DHT, for P2P IR. The new index uses an algorithm similar to Huffman coding with a modification to the DHT structure based on the term distribution. In a Huffman-DHT, a frequent term is assigned to a short ID and allocated a large space in the node ID space in DHT. Throuth ID management, the Huffman-DHT balances the index registration accesses among peers and reduces load concentrations. Huffman-DHT is the first approach to adapt concepts of coding theory and term frequency distribution to load balancing. We evaluated this approach in experiments using a document collection and assessed its load balancing capabilities in P2P IR. The experimental results indicated that it is most effective when the P2P system consists of about 30,000 nodes and contains many documents. Moreover, we proved that we can construct a Huffman-DHT easily by estimating the probability distribution of the term occurrence from a small number of sample documents.

Teeth segmentation in computed tomography (CT) images is a major and challenging task for various computer assisted procedures. In this paper, we introduced a hybrid method for quantification of teeth in CT volumetric dataset inspired by our previous experiences and anatomical knowledge of teeth and jaws. In this regard, we propose a novel segmentation technique using an adaptive thresholding, morphological operations, panoramic re-sampling and variational level set algorithm. The proposed method consists of several steps as follows: first, we determine the operation region in CT slices. Second, the bony tissues are separated from other tissues by utilizing an adaptive thresholding technique based on the 3D pulses coupled neural networks (PCNN). Third, teeth tissue is classified from other bony tissues by employing panorex lines and anatomical knowledge of teeth in the jaws. In this case, the panorex lines are estimated using Otsu thresholding and mathematical morphology operators. Then, the proposed method is followed by calculating the orthogonal lines corresponding to panorex lines and panoramic re-sampling of the dataset. Separation of upper and lower jaws and initial segmentation of teeth are performed by employing the integral projections of the panoramic dataset. Based the above mentioned procedures an initial mask for each tooth is obtained. Finally, we utilize the initial mask of teeth and apply a variational level set to refine initial teeth boundaries to final contour. In the last step a surface rendering algorithm known as marching cubes (MC) is applied to volumetric visualization. The proposed algorithm was evaluated in the presence of 30 cases. Segmented images were compared with manually outlined contours. We compared the performance of segmentation method using ROC analysis of the thresholding, watershed and our previous works. The proposed method performed best. Also, our algorithm has the advantage of high speed compared to our previous works.

It is known that the schedulability of a non-preemptive task set with fixed priority can be determined in pseudo-polynomial time. However, since Rate Monotonic scheduling is not optimal for non-preemptive scheduling, the applicability of existing polynomial time tests that provide sufficient schedulability conditions, such as Liu and Layland's bound, is limited. This letter proposes a new sufficient condition for non-preemptive fixed priority scheduling that can be used for any fixed priority assignment scheme. It is also shown that the proposed schedulability test has a tighter utilization bound than existing test methods.

Scanning packet payload at a high speed has become a crucial task in modern network management due to its wide variety applications on network security and application-specific services. Traditionally, Deterministic finite automatons (DFAs) are used to perform this operation in linear time. However, the memory requirements of DFAs are prohibitively high for patterns used in practical packet scanning, especially when many patterns are compiled into a single DFA. Existing solutions for memory blow-up are making a trade-off between memory requirement and memory access of processing per input character. In this paper we proposed a novel method to drastically reduce the memory requirements of DFAs while still maintain the high matching speed and provide worst-case guarantees. We removed the duplicate transitions between states by dividing all the DFA states into a number of groups and making each group of states share a merged transition table. We also proposed an efficient algorithm for transition sharing between states. The high efficiency in time and space made our approach adapted to frequently updated DFAs. We performed several experiments on real world rule sets. Overall, for all rule sets and approach evaluated, our approach offers the best memory versus run-time trade-offs.

In PKC 2004, Choi et al. proposed an ID-based authenticated group key agreement (AGKA) protocol using bilinear pairings. Unfortunately, their protocol suffered from an impersonation attack and an insider colluding attack. In 2008, Choi et al. presented an improvement to resist insider attacks. In their modified protocol, they used an ID-based signature scheme on transcripts for binding them in a session to prevent replay of transcripts. In particular, they smartly used the batch verification technique to reduce the computational cost. In this paper, we first show that Choi et al.'s modified AGKA protocol still suffers from an insider colluding attack. Then, we prove that the batch verification of the adopted ID-based signature scheme in their modified protocol suffers from a forgery attack.

Bessel beams are a family of diffraction-free beams. They have many unique properties and prospective applications. Much attention has been focused to this subject in optics. Recently, the studies of such beams at mm- and submm- wavebands have been carried out in our group. The investigation results, including their theories, generation, propagation and potential applications, are presented in this paper.

We investigate numerically the applicability of photonic crystal fiber (PCF) with a uniform air hole structure as a wide-band transmission medium. We show that accumulated dispersion over the PCF can be reduced effectively by optimizing the index profile of dispersion compensating fiber (DCF). We also show that a bandwidth of more than 300 nm will be available for 40 Gbit/s NRZ transmission by using the PCF as a transmission medium instead of conventional 1.3 µm zero-dispersion single-mode fiber (SMF).

We have developed a novel downlink packet scheduling scheme for a multiuser OFDMA system in which a subchannel can be time-multiplexed among multiple users. This scheme which is called Matrixed-based Proportional Fairness can provide a high system throughput while ensuring fairness. The scheme is based on a Proportional Fairness (PF) utility function and can be applied to any of the PF-based schedulers. Our scheduler explores multichannel multiuser diversity by using a two-dimensional matrix combining user selection, subchannel assignment, and time slot allocation. Furthermore, unlike other PF-based schemes, our scheme considers finitely backlogged queues during the time slot allocation. By doing so, it can exploit multichannel multiuser diversity to utilize bandwidth efficiently and with throughput fairness. Additionally, fairness in the time domain is enhanced by limiting the number of allocated time slots. Intensive simulations considering finitely backlogged queues and user mobility prove the scheme's effectiveness.

This paper proposes a power saving control function for battery-powered portable wireless LAN (WLAN) access points (APs) to extend the battery life. The IEEE802.11 standard does not support power saving control for APs. To enable a sleep state for an AP, the AP forces the stations (STAs) to refrain from transmitting frames using the network allocation vector (NAV) while the AP is sleeping. Thus the sleep state for the AP can be employed without causing frame loss at the STAs. Numerical analysis and computer simulation reveal that the newly proposed control technique conserves power compared to the conventional control.

The IEEE 802.16e standard specifies the sleep mode and the idle mode of a mobile station (MS) for power saving. In this paper, to reduce the energy consumption of the MS, we employ the sleep mode while the MS is on-session, and the idle mode while it is off-session. Under the assumption that the time duration from the end of a session to the arrival of a new downlink session request follows an exponential distribution of the mean and that arrivals of messages during an on-session follow a Poisson process with rate λ, we analyze the awake mode period and the sleep mode period by using the busy period analysis of the M/G/1 queue, and then we derive the total mean length of an on-session which consists of a geometric number of awake mode periods and sleep mode periods. Since the sum of an on-session and an off-session constitutes a cycle, we can express the average power consumption in terms of the mean lengths of an awake mode period, a sleep mode period and an idle mode period. The average power consumption indicates how much the MS can save energy by employing the sleep mode and the idle mode. We also derive the Laplace Stieltjes transform (and the mean) of the queueing delay of messages to examine a tradeoff between the power consumption and the delay of messages. Analytical results, which are shown to be well-matched by simulations, address that our employment of the sleep mode and the idle mode provides a considerable reduction in the energy consumption of the MS.

This paper presents a new cycle time synchronization method to transmit isochronous multimedia data by real time in IEEE 1394 over a UWB (ultra wide-band) network. The 1394 TA recommended two methods for the cycle time synchronization. The first method must use two consecutive beacon signals to calculate a drift correction, while the second one eliminates this dependency with minor algorithm changes. As experimental results, the second method achieves 21% performance improvement over the first one. The receipt of two consecutive beacons every time is hard due to the noise in a wireless channel. In addition, this paper provides the procedure of cycle time synchronization, as well as the transaction between 1394 protocol adaptation layer and IEEE 802.15.3 media access layer. The proposed synchronization technique will contribute to transfer isochronous data at IEEE 1394 over UWB audio/visual appliances such as camcorder, HDTV, etc.

This letter presents a new quadrature voltage-controlled oscillator (QVCO) consisting of two n-core Colpitts voltage-controlled oscillators (VCOs) with a tail inductor. The VCOs are used as a single-ended injected injection-locked frequency divider (ILFD). The output of the tail inductor in one ILFD is injected into the injection node in the other ILFD and vice versa. The proposed QVCO has been implemented in the 0.18 µm CMOS technology. At the supply voltage of 1.0 V, the power consumption is 1.8 mW. The free-running frequency is tunable from 4.68 GHz to 5.03 GHz as the tuning voltage is varied from 0.0 V to 1.8 V. The measured phase noise is -113.58 dBc/Hz at the 1 MHz frequency offset from the oscillation frequency of 5.03 GHz and the figure of merit (FOM) of the QVCO is -185.06 dBc/Hz.

We propose a novel 50 Mb/s optical transmitter fabricated in a 0.6 µm BiCMOS technology for automotive applications. The proposed VCSEL driver chip was designed to operate with a single supply voltage ranging from 3.0 V to 5.25 V. A fully integrated feedforward current control circuit is presented to stabilize the optical output power without any external components. The experimental results show that the optical output power can be stable within a 1.1 dB range and the extinction ratio greater than 14 dB over the automotive environmental temperature range of -40 to 105.

In this paper, we propose a predicate indexing method which handles equality and inequality tests separately. Our method uses a hash table for the equality test and a balanced binary search tree for the inequality test. Such a separate structure reduces a height of the search tree and the number of comparisons per tree node, as well as the cost for tree rebalancing. We compared our method with the IBS-tree which is one of the popular indexing methods suitable for data stream processing. Our experimental results show that the proposed method provides better insertion and search performances than the IBS-tree.

In this paper, we focus on resource allocation schemes for minimizing the energy consumption of subscriber stations (SSs) in uplink flows of the IEEE 802.16 OFDMA systems. The resource allocation schemes assign subcarriers, powers, and data rates to each SS based on the measured signal to noise ratio (SNR) of the uplink channel and predefined modulation and coding scheme as system parameters. Previous research efforts to optimize resource allocation focus on the rate and throughput maximizations, and develop suboptimal heuristic algorithms. However, this paper intends to reduce the energy consumption of SSs by considering the relationship between energy efficiency and resource allocation. In order to clearly formulate the relationship, we use the Multiple Choice Knapsack (MCK) problem, which is proved to be an NP-hard problem. We propose two heuristic schemes to solve the NP-hard problem, which adaptively use the modulation and coding scheme, defined in the IEEE 802.16 OFDMA systems to minimize the required transmission power of each SS. Our simulation results show that the proposed schemes can reduce the energy consumption by up to 53% compared to the channel state information (CSI) scheme, which determines the modulation and coding level only considering the channel state information.

The paper introduces both high-speed transmission and quality of system to offer the Internet services on a HFC (Hybrid Fiber Coaxial) network. This utilizes modulating the phase and the amplitude to the signal of the IPMS (Internet Protocol Multicasting Service). An IP-cable transmitter, IP-cable modem, and IP-cable management servers that support 30-Mbps IPMS on the HFC were developed. The system provides a 21 Mbps HDTV transporting stream on a cable TV network. It can sustain a clear screen for a long time.

This letter presents an adaptive beamformer robust to random steering errors, based on the projection of received signals onto the orthogonal complement of the interference subspace. In the presence of random steering errors, to prevent the suppression of the desired signal, the proposed beamformer effectively finds basis vectors for the estimation of the interference subspace.

Dual-band chip antennas usually have a narrow bandwidth in the first resonance frequency band due to an inter-coupling capacitance. In order to analyze the effect of the inter-coupling capacitance, an equivalent circuit of an antenna with a branch radiator is considered in this paper. Based on the equivalent circuit model, it is found that the inter-coupling capacitance reduces impedance bandwidth. This paper proposes a gap feeding method to alleviate the effect of the inter-coupling capacitance and explains it using an equivalent circuit.